With advances in electronic products, semiconductor technologies have been widely applied in manufacturing various memory devices, central processing units (CPUs), liquid crystal displays (LCDs), light emitting diodes (LEDs), laser diodes and a plethora of other devices. In order to achieve high integration levels and attain high-speed requirements, dimensions of the various features that make up semiconductor integrated circuits have been reduced and various materials such as copper and ultra low-k dielectrics, have been introduced. The incorporation of the new materials involves using new techniques that overcome manufacturing obstacles otherwise associated with the new materials. Today's rapidly progressing semiconductor manufacturing industry requires the continued development of such new materials and techniques.
Included among memory devices are volatile and nonvolatile memory devices. A volatile memory device such as a DRAM (dynamic random access memory) is provided to store data or information. A DRAM cell may include only a transistor and a capacitor. Due to its simple structure, costs for manufacturing DRAMs are low as the DRAM manufacturing process is relatively simple. When a voltage that had been applied to a DRAM is turned off, however, the data stored in the volatile DRAM cell disappears. Moreover, DRAM cells must be periodically refreshed to maintain the data stored therein because of current leakages such as from the capacitors in the DRAM cells.
Nonvolatile memory devices such as flash memory devices have been widely used as they maintain data even after input voltages have been removed. When desired, the data stored in the flash memory cells can be removed by UV radiation or electrical erasing. A flash memory cell, however, usually requires multiple gate structures for storing data and is more complex and difficult to manufacture than a DRAM cell. Therefore, the corresponding manufacturing cost for producing flash memory structures is comparatively high. Moreover, the erase/rewrite cycle of flash memory devices can be disturbed due to leakage currents from floating gates of the flash memory cell. As such, there are various shortcomings associated with nonvolatile devices such as a typical flash memory cell.
Recently, various other nonvolatile memory devices such as phase change random access memory (PCRAM) devices, magnetic random access memory (MRAM) devices and ferroelectric random access memory (FRAM) devices having cell structures more similar to those of DRAM devices, have been proposed and are being developed.
A PCM cell of a PCRAM generally includes a phase change element and a structure such as a transistor or other device that applies current to the phase change element. In one embodiment, one source/drain of the transistor may be coupled to ground with the other source/drain coupled to a phase change element and the transistor gate coupled to a gate voltage. Another portion of the phase change element may be coupled to a bit line voltage. According to this embodiment, when the data stored within the phase change element is to be accessed, a voltage is applied to turn on the transistor and the bit line voltage is applied to the phase change material such that a read current may flow through the phase change element and the transistor. Based on the level of output current, the data stored within the phase change element is accessed.
Using the aforementioned arrangement or other arrangements, the level of output current depends upon the phase and impedance of the phase change material. By changing the phase of a phase change material such as from amorphous to crystalline or vice versa, the impedance of the phase change material may dramatically change. The changing impedance of the phase change material enables the phase change material to store different data. For example, the low-impedance form of the phase change material may store a data value of “1” whereas the high impedance form of the phase change material may store a data value of “0.”
Based on the foregoing, PCM structures and methods for incorporating the same into semiconductor devices are desired.
The phase of the phase change material is advantageously changed by a heater plug or other device which contacts and heats the phase change element to change the phase thereof when a set current is applied to the heater. The set and reset currents applied to change the phase of the phase change material are strongly proportional to the contact area between the heater plug and the phase change material. This is shown in FIG. 1 which is a graph showing the relationship between a reset current, in mA, and the contact area, in nm2, between the heater and the phase change material. In order to be able to accurately and reliably control the phase change with a known current, it is important to reliably produce a uniform and desired contact area between the phase change materials and their associated heaters.
Based upon the foregoing, PCM cell structures with accurately produced and controlled contact areas and methods for forming the same, are desired.